Dual range exposure meter having a light-diffusing pivoted cover



April 18, 1950 2,504,346

D. w. NoRwooD DUAL RANGE EXPOSURE METER HAVING A LIGHT-DIEEUsING PIvoTED covER Filed July 17, 1948 BY/W/ Patented Aprs 18, 1950 l2,504,346 nUAL RANGE EXrjsURE METER HAVING A I" ltlorwood, Pasadena, l Applidationxuiy 1'7, 194s, serial No. '39,283

ctptance to ,-light, providedwith external 1 range-V changing means -serv-ingwto change the measurings range fof `-the :instrument f without,- affecting the angular light acceptance -1 orthe @device whereby the incident lightvalues obtained inthe different ranges are-'comparable With-each other ffm Photographic purposes v *Y A further bjeC?.,-0fg;the invention is to provide a device of the-.character described, which is'of. compact size and ymay.beprovided ,with a single light-intensity scale, r together with-range-chang. ing. means y adapted ,i to alter'- thel sensitivity ofl the;Y device inwsuchf mannerthat-different light intensities mayfgbe read-cn that scale subjectk to certaine preselected factor :such as 'f a factor often or-onefhundred.forthe readings Lin one.

ofthe ranges.- f 1 ,Ag particular object of the invention is to provide av dual range-exposure meter-adapted for measuring the-intensityfof light'incident upon va photographic subject` over a solidangle in the neighborhood of andv slightly greater vthan 21r steradians, iirwhichthe angleof light acceptancev of; 4the meter is substantially the sa-me for both ranges. i l. i'

`Eff-Llhis application is a continuation-impart of my :forfeited application Ser; No; 588,539, led' .-Other advantages and objects of the invention will be 4brought f out' in'the ensuingdescriptionv` or-E will bev apparentfrom suchf ldescription `iin',- conne'ction withV theaccornpanying v`drew'vings, in'

rwhich: i v

FFig.- 1 is a perspective view of the light receiving vend portion of alight meter in accordance with this invention, Awith' the Vrange-changing' cover memberin closed position ;A

Tig. 2 is a planview thereof, as viewed from aboveinFigLl'; f

'y Ci'a'im (C12 884.23)

Fig. 3 is a sectional view on linev 3-3 in Fig. 2; and

Fig. 4 is a sectional view on line 4 4 in Fig. with the cover'memb'1g-ii*fshovv'n4 in ope "'psil ion. a 11.1". ciw..

yThe device is shown as comprising a casing 3i of any desired"shapej-'providd-et n'd with a recess 32 inwhich the light vresponsive element is disposed. Thecasing 3i isalsf'p'rovided lwith a' current lneasurifng'vf device loca-'ted Within the' casing;slichasfanibroamrnetenalid with a transparent n'dow '2fpe'rinittig the viewing of alight intensity scaleand the microammeterpointer'lfif ne Alight responsive element 3v3is mounted Within the recess 3.2T'adjaceiit1 'said one fend:g `f v'the casing,V and'is provided Witl'ilan upper infront electrode 34 engaging the iront? surface of said element at one end thereof, "anda 'levi/er or' back'l electrode '35,preferablybfa spring type, engag-y ingA the rear surface of said elei'nnt,said`elec trodes being connectedto the current generating device asis common' in exposure 'meters l"employe ing photo'sensitive cells; 'Y

' -Overlying the 'light responsive elementv 33.. I

have provided'a translucent Ylight'-diffusing win-` n.; the walls of recess32 andt engaging cooperating' Ilanges 39 on the Window' 36. "The anges 38" @surface 33a of light responsiveelement, loe;

dow member 3 whose inner "andouterffacesf'iiave a slight outwardly convex curvature. rSai'ol'zwin'- dow 36 may, foi-"example, beir'med otranslucentlight diffusing plastic." su'ch asw milk' cel# luloid or the like, or: of diffusing glass`,faiid"pref erably has a relativelyvhigh'light-transniitting power, for example; ai 50%.liglit:transmission characteristic, and may be heldin position' by. a ange e8-on casing 3| 'projecting inwardly fromV define a central Opening' 4l `at the endf'ofath'e casing, within Whichthe fwind'ow, 36 'is' disposed; so that the outer surface "of Window 36 is adaptedt to receive light incident thereon in directions throughout a solid angle in the neighborhood of and-slightly greater than21r steradians. The

light'so received bysaidwindow-is diffused and.

transmitted inwardly to the outer light-receiving cated inwardly ofthe anses .38v and `the front electrode 34.; f

In order to permit assembly of the device with the-window 36 secured in position as above dev vvscribed, `the'casing `3| fis preferably split longitudinally along =a centralfplane; asv indicated' at Bia, so as to provide two. casing halves which may be secured together byl lmean-s of retaining A range-changing cover 42 is hingedly mounted on the casing 3| by means of lugs 43 on the cover and lugs 44 on the casing, pivotally interconnected by hinge pin 45. Said cover `42 is mounted at the same end of the casing as the window 36, and is movable about the hinge axis denned by pin 45, to a closed or operative position overlying said window, as shown in Figs. 1 through 3, and to open or inoperative position away from said window, as shown in Fig. 4. The angular movement of said cover away from its closed position permitted by the hinged mounting thereof is preferably sufficient so that, when in the full open position of Fig. 4, the cover is entirely removed from the solid angle of light acceptance of the curved window 36, whereby said window is exposed to reception of exterior light in directions throughout the aforesaid solid angle slightly greater than 21r steradians.

In order to releasably retain the cover 42 in either full closed or full open position, I have shown the casing as provided with latch means in the form of a spring-pressed ball 46 adapted to engage a small notch 41a in one of the hinge lugs 43 when the cover is in closed position, and to engage a notch 41h in said lug, as shown in Fig. 4, when the cover is in its full open position.

The cover 42 may be of metal, plastic, or other opaque material and is provided with a central opening 49 of substantially the same size and shape as the opening 40 in the casing. A translucent light-diffusing window of substantially the same shape, area, and convex surface configuration as the fixed window 36, is iitted within said opening 49 in the cover, and may be provided with a peripheral iiange 52 engaging a recess in said cover around said opening. The window 5| is secured to said cover in any suitable manner, as by cementing it thereto. The opaque cover 42 extends completely around the opening 49 and window 5| and is adapted to closely engage the end wall of casing 3| when in closed position so as to prevent access of exterior light to the inner window 36 between said cover and the casing and thereby restrict illumination of the light-responsive element 33 to light passing through the outer window 5|.

The window 5| may, for example, be also formed of milk celluloid or other translucent light-diffusing plastic material or of diffusing glass, having a light-transmitting power sunciently low to reduce the indications of light quantity to the desired value when the cover is in closed position. For example, if it is desired to reduce the indications of light quantity, with the cover closed, to one-tenth the indications produced by light of the same intensity when the cover is open, the window 5| should be made to have a light-transmission characteristic of In one particular form of this device, each of the windows 36 and 5| is curved convexly in each of two planes that are perpendicular to each other and perpendicular to the surface of the Window at the center of the Window, said planes passing through the longitudinal and transverse central axes of the windows, and the curvature in each of these planes is such as to subtend an angle of approximately 12 at the center of curvature. With this arrangement, the outer surface of each window is adapted to receive light incident thereon in directions throughout a solid angle of approximately 2.21r steradians.

The device may Ibe used for taking incident light measurements in either of two different ranges of light intensity, with the cover member 4 42 in fully open position and fully closed position. respectively.

When the cover 42 is in the fully open position shown in Fig. 4, the window 36 is open to access of exterior light incident thereon in directions throughout the above mentioned solid angle somewhat greater than 211 steradians, and such incident light is transmitted to the full area of the light-receiving surface 33a of the light-responsive element 33, causing said element to produce a relatively large indication of light quantity by the associated microammeter pointer, for a given intensity of illumination. When the cover 42 is swung to its fully closed position overlying the window 36, the outer window 5| will receive light incident thereon in directions throughout the same solid angle as before, and will transmit a given portion of such light through the inner window 36 to the full light receiving area of the light-responsive element 33. The proportion of the light so transmitted inwardly by the outer window 5| is determined by its light-transmitting' power, which as noted above is so selected as to give the desired reduction in the indications of light quantity when the cover is closed, as compared with the indications for the same incident light intensity with the cover open. v

In the specific example mentioned above, in which the outer window 5| has a light-transmitting characteristic of 10%, the indications obtained with the cover closed will be one-tenth as great as with the cover open, for the same intensity of illumination. Thus, if the device is made to provide indications of illumination in the range from 2.5 to 1280 foot-candles when the cover is open, it may be used to measure lightl values in the range from 25 to 12,800 foot-candles when the cover is closed.

The light on a photographic subject is provided not only by the direct light of the main light source but by light from other directions which reinforces the illumination of the portions of the subject which are facing the camera. In using the incident light meter of this invention, the effectiveness of this illumination for photographic purposes is determined by measuring the light falling upon the light-receiving surface of the light-responsive element when the device is located at the position of the photographic subject with the light-receiving end of the device (i. e., the upper end as shown in the drawings) directed toward the position of the main source of illumination. Since the two windows 36 and 5| are of the same size and convex curvature, and since f both of these windows are translucent light-diffusing members and are so positioned as to transmit the incident light inwardly to the full lightreceiving area of the light-responsive element 33,

it will be seen that the angular light-receiving characteristic of the instrument is maintained equal and slightly greater than 21r steradians inY both the open and closed positions of the movable cover member 42. I have found that this ability of the device to receive and measure the incident illumination over this wide angle of acceptance in both the high-intensity and low-intensity ranges is advantageous in that it provides a more satisl factory measurement oi the total illumination intensity, including not only the light from the main source of illumination but also light from other sources that assist in illuminating the portion of a photographic subject which is directed toward the main source of illumination.

I claim:

An exposure meter for measuring the intensity of incident light in two diierent ranges of light intensity, comprising: a casing provided with a first translucent light-diffusing window at one end thereof; a light-responsive element within said casing adjacent said one end and having a light-receiving surface positioned to receive light transmitted by said Window; said window having a slight outwardly convex curvature and being so positioned with respect to said casing and said light-responsive element as to receive, and transmit to substantially the full area of said light-receiving surface, exterior light incident upon said Window in directions throughout a solid angle in the neighborhood of and slightly greater than 2W steradians; an indicating member associated with said light-responsive element for producing a visible indication ci' the quantity of light incident upon said 1ight-receiving surface of the light-responsive element; an opaque cover hingedly mounted on said casing for movement to a nrst position covering said Window and a second position substantially fully exposing said window to the reception of light throughout said solid angle, said cover being provided With a second translucent light-diffusing window disposed outwardly of said rst Window when said cover is in said rst position; said second window having its outer surface disposed substantially at the outer face of said cover and having substantially the same shape, area and convex curvature as said first window and being so positioned as to receive exterior light incident thereupon in directions throughout substantially the same solid angle in the neighborhood of and slightly greater than 21r steradians, and to transmit a portion of all such incident light through said rst Window to substantially the full area. of said light-receiving surface of the light-responsive element; said cover engaging said casing around said rst Window when in said rst position in such manner as to substantially prevent access of exterior light to said rst window otherwise than by transmission through said second Window; and said second window having a sufficiently low light-transmitting power to reduce the indication of light quantity produced by said light responsive element and the associated indicating member when said cover is in said first position to a value materially less than that produced by exterior light of the same intensity when said cover is in said second position.

DONALD W. NORWOOD.

REFERENCES CITED The following references are of record in the hie of this patent:

UNITED STATES PATENTS Number Name Date 1,990,361 Bailey Feb. 5, 1935 2,143,500 Smethurst Jan. 10, 1939 2,156,734 McCune May 2, 1939 2,274,441 Williams Feb, 24, 1942 2,278,338 Tonnies Mar. 31, 1942 FOREIGN PATENTS Number Country Date 416,553 Great Britain Sept. 17, 1934 

